The present invention relates to a variable gain control circuit.
A variety of variable gain control circuits are known which are designed to vary the gain by the control signal. S. Tanaka et al. "Non-linearity Improvement of Variable Gain Circuit For T.V. Camera Use", Institute of Television Engineers (ITE) Conference, Nov. 29, 1978, TBS50-1, for example, discloses a circuit in which the control voltages applied to the bases of a pair of transistors are varied, thus changing the collector current of either transistor and thus varying the gain of the circuit. The ratio of the collector current of one of the transistors to the collector current of the other transistor non-linearly changes because each transistor has a base resistance. Both the differential gain characteristic and differential phase characteristic of this variable gain control circuit are inevitably not so good.
The thesis identified above further discloses the technique of connecting a positive feedback loop with a feedback factor G.multidot..beta.=1/2 to the bases of a pair of transistors to eliminate the influence of the base resistance of either transistor. The thesis, however, does not teach how to apply the technique to a variable gain control circuit.
The known variable gain control circuits have another drawback. In each of them, when rectangular wave control signals are supplied to a pair of transistors, thus controlling the switching operation of the transistors, the switching speed of one transistor which is cut off is inevitably lower than that of the other transistor which is not cut off. Consequently, the circuit generates an output signal containing a glitch.